Fixing device, fixing method, and recording apparatus

ABSTRACT

A fixing device for fixing the toner on the recording medium comprising a pair of fixing rollers which face each other and rotate in opposite direction to form a nipping and fusing section for toner fixing, with at least one of the rollers internally provided with a heating element, and a recording medium guiding device for changing the length of contact area of the recording medium to the fixing roller with heating element in accordance with the recording medium thickness, which is provided at the downstream side extending from the nipping and fusing section. This allows the device to be reduced in size and the fixing strength to be increased independent of the recording medium thickness.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a recording apparatus based onelectrophotographic method for a printer, copying machine, facsimile, orthe like, and, more particularly, to a fixing device and fixing methodexcellent for fixing a toner image onto a recording medium, and arecording apparatus.

2. Description of the Related Art

A conventional recording apparatus based on electrophotographic methodis involved in a process for making visible a toner image on a recordingmedium, and in a process for fixing the resulting toner image onto therecording medium. The heated toner image is fused onto the recordingmedium, and then cooled and solidified. In the fixing process, utilizingthe properties of the toner, it is heated and fused, and then naturallycooled so that it is fix onto the recording medium.

In a conventional fixing device, in the fixing process, a pair of fixingrollers, made up of one roller and one supporting roller, ispress-contacted. At least one of the fixing rollers is heated to bringthe pair of fixing rollers into contact with each other, thereby forminga nipping and fusing section. The recording medium is passed throughthis nipping and fusing section to fix the toner aligned thereon. Whenthe recording medium is passed through the nipping and fusing section,the aligned toner which forms an image on the recording medium is heatedand at the same time subjected to pressure. The heat energy and pressureapplied at the nipping and fusing section changes the shape of thetoner. This action causes the toner to be fixed onto the recordingmedium. Here, only one of the fixing rollers needs to be heated. Thetoner image formed on the recording medium passes through the nippingand fusing section so that its toner-image bearing surface and theheated fixing roller contact each other. The heated heat fixing rolleris called the heat roller, while the supporting roller is called theback-up roller. The heat roller and the back-up roller together may becalled a pair of fixing rollers. In addition, either of the heat rolleror the back-up roller may be called a fixing roller.

The heat roller is a hollow cylinder made of aluminum and has a heaterat its central section. The aluminum hollow cylinder is called a coremetal. A halogen lamp is often used for the heater. When this fixingdevice is used to heat the toner image, offsetting may occur in whichthe fused toner gets stuck onto the heat roller. When fixing is carriedout for the second time, offsetting causes the toner to be transferredfrom the heat roller onto the recording medium, which results inmisprinting. To prevent this, a material having high releasingcapability such as silicone, rubber, fluorocarbon rubber, orfluorocarbon resin is used to form a parting layer, which is the surfacelayer of the heat roller which contacts the toner. In general, fluorine,silicone rubber, or fluorocarbon rubber is used for the surface layer.In particular, fluorocarbon resin is widely known for its high releasingcapability. Examples of fluorocarbon resin surface layer materials whichare frequently used include perfluoroalkoxy copolymer (PFA) andpolytetrafluoroethylene (PTFE). The parting layer, which has a lowerthermal conductivity than the core metal, acts as a thermal resistancesection during fixing.

On the other hand, the back-up roller or the supporting roller has anelastic layer formed along the outer periphery of its metallic rotatingbody. When it is press-contacted with the heat roller, the elastic layerchanges shape to form a nipping and fusing section.

Fan fold paper is primarily used for the recording paper. In thelongitudinal dimension, it has perforations spaced apart at certainintervals. This type of recording medium is also called continuouspaper.

In a conventional method of fixing a toner image by transportingcontinuous paper at a high Speed, the pair of fixing rollers do notprovide enough heat for the toner. Therefore, a preheater or other suchmeans is placed at a location just before the recording medium passesthrough the pair of fixing rollers. This preheater is often a hot platewhich is positioned in the transporting path. It performs preheatingfrom the back surface of the toner-image bearing surface of thecontinuous paper, which makes it necessary to supply heat correspondingto the thickness of the recording medium. This is not thermallyefficient. The preheater, itself, must be made larger for sufficient andeffective preheating, with the result that more space must be providedfor the preheater than for the pair of fixing rollers. Such a prior artdescribed above is disclosed, for example, in Japanese Patent Laid-OpenNo. Sho 64-9483 and Japanese Patent Laid-Open No. Hei 25-504634.

Merely removing the preheater does not solve the above-described problempresent in the conventional fixing device. This is because it isdifficult to provide the necessary heat for fixing the toner image byonly using a pair of fixing rollers alone. When the parting layer of thebeat roller is reduced in thickness, its thermal resistance decreases.As the amount of heat applied to such a thin parting layer increases,the wearing resistance of the heat roller is considerably reduced. Thisin turn reduces the heat roller life, so that the fixing device cannotbe used for practical purposes. In preheating the recording medium whichis brought into contact with the heater before it reaches the nippingand fusing section or forwardly of the heat roller, the toner is onlyaccumulated on the recording medium and is not adhered thereon. Inaddition, the recording medium, which is not nipped by the back-uproller, undergoes thermal contraction. This causes a slippage betweenthe recording medium and the heat roller surface, giving rise to imageslippage.

To solve the above-described problems, a method may be used to allow therecording medium to remain in contact with the heat roller even after ithas passed through the nipping and fusing section. Since the toner imageis heated while it is nipped between the rollers, it is adhered onto therecording medium without the occurrence of image slippage. The conditionof the toner image immediately after it has been nipped is notsatisfactory from the viewpoint of fixing because there is not enoughadhesion force. However, even if, a slippage occurs between therecording medium and the heat roller surface due to thermal contractionof the medium, image slippage will not occur since there is someadhesion force between the toner and the recording medium. Since thermalcontraction of the recording medium is considerable immediately afterheating, a substantial amount of thermal contraction force is developedin the nipping and fusing section. However, the medium is nipped by theback-up roller, so that thermal contraction, itself, does not occur.Thermal contraction of the recording medium is slight in the contactregion after it has passed through the nipping and fusing section. Whenthis method is taken, the following technical problems need to beovercome.

(1) For a thick recording medium, the same conventional method is usedto increase the amount of heat provided by the heat roller, which isrequired for overcoming fixing failure. When the thick medium issubjected to the same heating conditions as a thin one, the toner isfixed less strongly onto the recording medium. Therefore, one technicalproblem which needs to be solved is how to increase the fixing strengthof the toner onto the recording medium irrespective of the mediumthickness.

(2) The same conventional method is used to provide more heat to thecentral portion of the heat roller as compared to the end portions ofthe heat roller extending beyond the width span of the: recordingmedium. The difference between the amount of heat supplied to thecentral portion and that supplied to the ends gradually increases, sothat when printing is performed successively for a long period of time,the temperature of the central portion of the heat roller dropssignificantly. This results in a marked temperature differencedistribution in the longitudinal direction of the heat roller. For thisreason, a fixing failure occurs, in particular, at the central portionof the heat roller, in which the toner is less strongly fixed onto therecording medium. Consequently, in carrying out printing successivelyfor a long period of time, another technical problem is how to controltemperature drop of the central portion of the heat roller to preventfixing failure thereof.

(3) After thermal energy and pressure have been applied to the recordingmedium and toner at the nipping and fusing section, heat energy issupplied from the contact region thereto. For this reason, more work isdone by thermal energy required for fixing by proportion than bypressure. As a result, after fixing, more time is required for thetemperature drop of the recording medium, which slows down thesolidification of the toner. Because of this, the toner remains fusedeven in a paper stacker which stacks recorded paper, so that when therecording medium is folded a phenomenon called toner stick occurs inwhich the toner adheres onto other portions of the recording mediumsurface not requiring fixing, and other toner images. When toner stickoccurs, the necessary information is transferred onto other recordingmedium surfaces, so that after recording unnecessary information may berecorded or necessary information may be missing to give rise to imageloss. Another technical problem is how to prevent misprinting and imageloss due to toner stick.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a fixing device havinga small-sized heat roller having a long life, and which prevents imageslippage from occurring, which is made possible by removing thepreheater.

Another object of the present invention is to allow the toner to befixed with sufficient strength onto the recording medium regardless ofthe medium's thickness.

Still another object of the present invention is to allow thetemperature drop of the central portion of the heat roller in the fixingdevice to the controlled when printing is performed successively for along period of time to prevent fixing failure thereof.

A further object of the present invention is to prevent misprinting andimage loss caused by toner stick from occurring.

To these ends, there is provided according to one aspect of theinvention a fixing device in which a recording medium to be transportedis nipped to fix the toner image formed thereon. The device comprises apair of rotatable rollers which face each other, with at least one ofthe rollers internally provided with a heating element, the pair offixing rollers press-contacting the recording medium to form a nippingand fusing section for fixing the toner image. It also comprises arecording medium guiding means for changing the length of the contactarea of the recording medium to the outer periphery of the fixing rollerwith heating element in accordance with the recording medium thickness,which is provided at the downstream side from the nipping and fusingsection formed by the fixing rollers. The recording medium guiding meansis made up of a recording medium guiding roller and a rotatablesupporting arm and the rotating angle of its supporting arm can be setto a predetermined value in accordance with the recording mediumthickness. In addition, a control means may be provided forautomatically setting the rotating angle of the supporting arm inaccordance with the recording medium thickness.

The fixing roller with heating element is an aluminum hollow cylinder,which is called a core metal, whose outer periphery is coated with aparting layer of fluorocarbon resin material, with a halogen lamp usedfor the heating element.

The fixing roller with hearing element may have at least one heat pipeas heating element embedded in the interior of the cylindrical coremetal in the longitudinal dimension.

The fixing roller with heating element may have in its interior aplurality of heating elements each having a different heating valuealong its longitudinal dimension and detectors for detecting the surfacetemperature of the roller, which are provided so as to face the rollerat positions corresponding to those sections of the heating elementswhere the heat value of each heating element is maximum, so as tocontrol the operation of the heating elements and thereby to maintainthe roller surface temperature at a predetermined value.

A circuitous guiding member may be provided along with the recordingmedium guiding means at the downstream side of the transportingdirection of the recording medium between the recording medium guidingmeans and a recording medium stacker for allowing cooling of therecorded medium.

The fixing roller with heating element may be a copper hollow cylinderor core metal whose outer surface is plated with nickel, the resultingouter periphery of the nickel plated surface further coated with aparting layer of fluorocarbon resin material,

Any of the aforementioned fixing device is built in the recordingapparatus as at least one component.

To these aforementioned ends, there is provided according to anotheraspect of the invention a fixing method in which a recording medium tobe transported is nipped for fixing the toner image adhered thereon. Thefixing method comprises the steps of fusing the toner bypress-contacting the recording medium between a pair of rotatablerollers facing each other with at least one of its rollers internallyprovided with a heating element; and changing the length of the contactregion of the recording medium to the outer periphery of the fixingroller with heating element in accordance with the recording mediumthickness by means of a recording medium guiding means provided at thedownstream side following the nipping and fusing section formed by thefixing rollers.

The fixing method may comprise the step of allowing cooling of therecorded medium by providing a circuitous guiding member between therecording medium guiding means and a recording medium stacker into whichis discharged recorded medium.

According to the present invention, more heat is applied to the toner byallowing the recording medium to remain in contact with the contactregion even after it has passed through the nipping and fusing section.

Sufficient fixing strength of the toner may be provided when a thickrecording medium is used. This is made possible by increasing thecontact region of the recording medium to the heat roller after it haspassed through the nipping and fusing section, whereby the proper amountof heat is supplied to obtain the right fixing strength.

Increasing the thermal conductivity of the core metal of the heat rolleraccelerates thermal dispersion in the longitudinal dimension of the heatroller. This makes uniform the temperature distribution of the heatroller in the longitudinal dimension, so that when printing is performedsuccessively for a long period of time, temperature drop at the centralportion of the heat roller is controlled. This prevents fixing failurefrom occurring at the central portion of the heat roller. In addition,the heater supplies more heat to the central portion, that is where moreheat is consumed, in accordance with the temperature distribution of theheat roller. This balances the amount of heat consumed and supplied,which makes uniform the temperature distribution of the heat roller inthe longitudinal dimension. This also prevents fixing failure fromoccurring at the central portion of the heat roller.

Purposely increasing the length of time the recording medium needs toreach the stacker after it has passed through the nipping and fusingsection and the contact region to the heat roller allows the recordingmedium to cool for that length of time. Therefore, the temperature ofthe recording medium in the stacker drops, so that toner stick will notoccur. Consequently, misprinting and image loss are prevented.

The objects, advantages, and novel features of the present inventionwill be described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of one embodiment of the presentinvention;

FIG. 2 is a cross sectional view of heat roller used in anotherembodiment of the present invention;

FIG. 3 is a cross sectional view of a heat roller used in anotherembodiment of the present invention;

FIG. 4 is a perspective view of a heat roller used in another embodimentof the present invention;

FIG. 5 is a longitudinal sectional view of another embodiment of thepresent invention; and

FIG. 6 is a longitudinal sectional view of another embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described with referenceto FIG. 1. FIG. 1 illustrates a fixing device in which a pair of fixingrollers 1 and 3 are press-contacted with each other, and in which atleast one of the fixing rollers 1 is heated to form a nipping and fusingsection 8. A recording medium 4 is passed through the nipping and fusingsection 8 to fix an unfixed toner image 5 thereon. A contact area 9extends from the nipping and fusing section 8. This is the region wherethe recording medium contacts the heated fixing roller (heat roller 1)after it has passed through the nipping and fusing section 8. Arecording medium guiding means 10 is provided for changing the length ofthe contact area 9 according to the thickness of the recording medium 4.The recording medium 4 and the unfixed toner 5 aligned thereon arepassed along a paper guide 11 through the nipping and fusing section 8,formed by press-contacting the back-up roller 3 and the heat roller 1,which has been heated by a heater 2. At the nipping and fusing section8, after receiving thermal energy and being subjected to pressure, theunfixed toner 5 is compressed, fused, and deformed. At the exit of thenipping and fusing section 8, the unfixed toner 5 becomes a semi-fixedtoner 6. When it is in a semi-fixed state, there is not enough fixingstrength. Thereafter, however, an adhesion force is developed betweenthe semi-fixed toner 6 and the recording medium 4, by an amount whichdoes not cause image slippage at the contact area where the recordingmedium contacts the heat roller 1. That is, the recording medium 4 andthe semi-fixed toner 6 formed on its surface are led by the recordingmedium guiding member 10 to the contact area 9, and further receivethermal energy from the heat roller 1 at the contact area 9, so thatthere is enough fixing strength. As a result, a fixed toner 7 havingexcellent fixing strength can be provided without the use of apreheater.

When fixing is experimentally performed with the recording mediumguiding member 10 removed and without the contact area 9, unlesstransference of the toner onto the heat roller 1 from the recordingmedium 4 or offsetting occurs, the toner is in a semi-fixed state to anextent where toner image slippage does not occur. When offsetting occursat the exit of the nipping and fusing section 8, not only does imageslippage occur at the contact area 9. The offset toner, which does notreturn to the recording medium 4 side, also remains on the heat roller 1surface, since pressure is not applied at the contact area 9. For thisreason, it is important to prevent offsetting from occurring.

When a thick recording medium is used, it is necessary to increase theamount of heat supplied by the heat roller to prevent fixing failure. Itis evident that performing the same heating as that for a thinnerrecording medium reduces to the fixing strength of the toner to therecording medium. This means that the fixing strength of the toner tothe recording medium must be increased in accordance with the recordingmedium thickness. The recording medium guiding member 10 of the fixingdevice of the embodiment can be rotated clockwise in the direction ofthe arrow A as shown in FIG. 1. The set angle θ of the recording mediumguiding member 10 is 90° when the recording medium 4 weighs 55 kg, 80°when it weighs 70 kg, 70° when it weighs 90 kg, 55° when it weighs 110kg, and 450 when it weighs 135 kg. Here, the weight of the recordingmedium is per 1000 sheets of A1-sized recording medium. By inputting thepaper thickness used for printing into the recording apparatus, the usercan allow the recording medium guiding member 10 to automatically set θto a predetermined angle. In addition, a detector for detecting thethickness of the recording medium 4 may be provided forwardly in thetransporting direction of the recording medium 4 from the fixing device,so that the set angle θ is automatically controlled in accordance withthe detected thickness. This operation provides the necessary andoptimal amount of heat necessary for sufficient fixing strength to thesemi-fixed toner 6. It is effective in consistently providing sufficientfixing strength in accordance with the recording medium 4 thickness.

Another embodiment of the invention will be described with reference toFIG. 2. FIG. 2 is a cross sectional side view of the heat roller 1 usedin the embodiment. Reference numerals 12 and 13 denote respectively aparting layer and a core metal. The other reference numerals denote thesame things as those in the first embodiment the core metal 13 is ahollow cylinder made of copper. Its outer diameter is 90 mm, while itsinner diameter is 75 mm. The longitudinal length as measured along adimension perpendicular to the plane of the drawing is 500 mm. Theparting layer 12 is made of PFA (fluorocarbon resin: perfluoroalkoxycopolymer), and is 60 μm in thickness. A halogen lamp is used for theheater 2, which shines on the core metal 13 and the parting layer 12 toheat them.

The central portion of the heat roller 1 experiences a largertemperature drop compared to both ends in the longitudinal directionwhich extend beyond the width span of the recording medium 4. Therefore,more heat is supplied to the central portion as in conventional fixingdevices. However, when all of the heat necessary for fixing is suppliedby the heat roller 1, more heat is supplied by the heat roller 1, sothat the difference in the amount of heat supplied to the centralportion as compared to the each end portion increases. When printing iscarried out successively for a long period of time, there is a largetemperature drop at the central portion of the heat roller 1. Thisdevelops a significant temperature difference distribution in thelongitudinal direction of the heat roller. As a result, fixing failuremay occur, in particular, at the central portion of the heat roller 1 inwhich the fixing strength of the fixed toner 7 to the recording medium 4is reduced. Therefore, in the embodiment, in performing printingsuccessively for a long period of time, temperature drop in the centralportion of the heat roller 1 is controlled to prevent fixing failurethereof, in the following way. For the core metal 3, there is usedcopper which has about three times the thermal conductivity as comparedto the conventionally used aluminum. This considerably improves thethermal diffusion of the heat roller 1 in the longitudinal direction.When printing is performed successively for a long period of time withthe conventionally used aluminum core metal, there is a temperaturedifference of about 30° C. between the central portion and both ends ofthe heat roller 1. When it is carried out with copper, as in theembodiment, the temperature difference can be reduced to about 15° C.This prevents fixing failure or reduction of the fixing strength of thefixed toner 7 to the recording medium 4 of the central portion of theheat roller 1 from occurring. Copper which is used for the core metal 13has poor adhesiveness to fluorocarbon resin PFA of which the partinglayer 12 is made, so that the heat roller 1 cannot be used for a longtime. To overcome this problem, in the embodiment, the heat roller 1 hasthe outer surface (not illustrated) of its copper core metal 13 platedwith nickel which has excellent adhesiveness to PFA to form a PFAcoating on the parting layer 12. According to the embodiment, there canbe provided a fixing device which is capable of preventing fixingfailure at the central portion of the heat roller 1, and which has along life because it has good adhesiveness between the core metal 13 andthe parting layer 12 of the heat roller 1.

Another embodiment of the present invention will be described withreference to FIG. 3. FIG. 3 is a cross sectional side view of the heatroller 1 used in the embodiment. Reference numeral 14 denotes a heatpipe. Other reference numerals denote the same things as those in FIG.2. The dimensions of the heat roller 1 are the same as those of thesecond embodiment. However, the core metal 13 is made of aluminum andthe outer surface is not plated with nickel. A plurality of heat pipes14 are embedded in the core metal 13 for transferring heat from bothends to the central portion of the heat roller 1 in the longitudinaldirection. The thermal conductivity of the core metal 13 is 5 times thatof an aluminum core metal. In the embodiment, thermal diffusion in thelongitudinal direction of the heat roller 1 is greatly improved byembedding heat pipes 14 which transfer heat from each end of the coremetal 13 to the central portion. In carrying out printing successivelyfor a long period of time, for the conventional aluminum core metal, thetemperature difference between the central portion and the end portionsof the heat roller 1 is approximately 30° C. With the embodiment,however, this can be reduced to less than 10° C. According to theembodiment, fixing failure of the central portion of the heat roller 1in which fixing strength of the fixed toner 7 to the recording medium 4is reduced, is made even less likely as compared to the secondembodiment.

Another embodiment of the present invention will be described withreference to FIG. 4. As the second and third embodiments, thisembodiment is aimed at preventing temperature drop of the centralportion of the heat roller and reduced fixing strength of the fixedtoner to the recording medium at the central portion of the heat rollerwhen successively carrying out printing for a long period of time.

FIG. 4 is a perspective view of the heat roller 1 used in the fourthembodiment of the present invention. Heater A 15 is a halogen lamp forincreasing the heating value of the central portion of the heat roller,while heater B 16 is a halogen lamp for increasing the heating value ofthe end portions of the heat roller. The heating value distributions areboth symmetrical with respect to the central line in the longitudinaldirection. The heat roller 1 is formed symmetrically with respect to thecentral line, so that the recording medium can be transported while thecentral lines of the medium and the heat roller 1 are aligned,regardless of the size of the medium. Temperature sensor A 17 detectsthe temperature of the outer surface of the central portion of the heatroller 1, while temperature sensor B 18 detects the temperature of theouter surface of the end portions of the heat roller 1. The heater A 15lamp is controlled such that it generates light when the temperaturesensor A 17 has detected that the temperature is below a predeterminedvalue. The heater B 16 lamp is controlled such that it generates lightwhen the temperature sensor B 18 has detected that the temperature isbelow: a predetermined value. The set predetermined temperatures forboth the temperature sensor A 17 and the temperature sensor B 18 are180° C. and are thus equal. Although the control system is not just theheater A 15 is controlled based on the detected temperature of thetemperature sensor A 17, while the heater B 16 is controlled based onthe detected temperature of the temperature sensor B 18, so that thecontrol systems are separately provided. The temperatures at two pointswhere the sensor A 17 for the central portion of the heat roller 1 isset and where the temperature sensor B 18 for the end portions of theheat roller is set can be controlled at 180° C. The heater roller 1allows the recording medium to be transported, while the central linesof the recording medium, which is symmetrically formed with respect tothe central line, and that of the heat roller 1 are matched. Thetemperature of the other end located symmetrically to the temperaturesensor B 18 can be controlled at 180° C., so that the temperaturedistribution of the heat roller 1 is made uniform. Supplying more heatto regions which consume a large amount of heat balances the amount ofheat consumed and supplied, thereby making uniform the temperaturedistribution of the heat roller in the longitudinal direction. Thiscontrols temperature drop of the central portion of the heat roller evenwhen printing is carried out successively for a long period of time,thereby preventing fixing failure of toner at the central portion of theheat roller. The effect of this embodiment has been described withreference to a symmetrical system along the longitudinal direction ofthe, heat roller. However, a non-symmetrical system may also be used. Inthe non-symmetrical system, heaters having a plurality of heat valuedistributions are used and temperature sensors are placed for eachheater for detecting the temperature of the heat roller surface. Each ofthe heaters are controlled separately.: This allows the same effects asthose of this embodiment to be realized.

Another embodiment of the present invention will be described withreference to FIG. 5. FIG. 5 is a cross sectional side view of the fixingdevice of the embodiment and the route taken by the recording mediumafter fixing. After passing the recording medium guiding member 10, therecording medium 4 is transported to a paper discharge stacker 20. Inthe stacker, the recording medium 4 is folded along its perforations andstacked thereat. After thermal energy and pressure have been applied atthe nipping and fusion section 8 to the recording medium 4 and toner,heat energy is supplied at the contact area 9 thereto. For this reason,more work is done by thermal energy required for fixing by proportionthan by pressure, with the result that after fixing the recording medium4 is overheated, which slows down the temperature drop of the medium.Along with this, the solidification of the fixed toner 7 is also sloweddown. Consequently, when the recorded medium 4, which has been subjectedto fixing, is discharged into paper discharge stacker 20 along theshortest distance, the recording medium 4 is folded in the dischargestacker 20 with its toner image in a fused state. This causes aphenomena called toner stick in which the fixed toner 7 sticks ontounspecified regions of the recording medium 4 or other recording medium,which has been subjected to fixing. When toner stick occurs, therequired information is transferred onto unspecified regions of therecording medium 4, so that after recording unnecessary misprintedinformation may occur in the output image, or image loss where necessaryinformation is missing may occur. To overcome this problem, in theembodiment, a circuitous guiding member 19 is provided, whereby afterpassing the recording medium guiding member 10, the distance oftransportation of the recording medium 4 to the discharge stacker 20 isincreased, thus giving the recording medium 4 time to cool. In general,although the paper can be cooled down about 50° C. every second bynatural cooling in air, once it is accumulated in the discharge stacker,thermal insulation of the paper itself drastically slows down thecooling speed to about 10° C. every second. Accordingly, increasing thetime for cooling in air by means of the circuitous guiding member 19allows sufficient cooling of the recording medium 4. When thetemperature of the recording medium 4 in the discharge stacker is lessthan 60° C., toner stick can be avoided. In the embodiment, therecording medium 4 has a temperature of 90° C. after it has passed therecording medium guiding member 10 and discharged into the dischargestacker 20 along the shortest distance. The circuitous guiding member 19allows the cooling time of the recording medium 4 to be increased by 0.7seconds even when it is discharged along the shortest distance. Thisreduces the temperature of the recording medium 4 to 55° C. in thedischarge stacker 20, thereby preventing toner stick from occurring.

Another embodiment of the prevent invention will be described withreference to FIG. 6. FIG. 6 is a cross sectional side view of the fixingdevice provided with a control means of the embodiment. The Figure showsan angle adjustment device 21, a control circuit 22, and a detector 23for detecting the recording medium thickness. Other members andreference numerals used are the same as those in the embodimentillustrated in FIG. 1. In addition, the operation and the function arethe same. The embodiment is constructed as having a control means forchanging the length of the contact area 9 where the recording mediumcontacts the heat roller in accordance with the recording medium 4thickness. The angle adjustment device 21 is formed by a servomotor,which turns clockwise or counterclockwise based on the signal of thecontrol circuit 22. The rotating shaft of the angle adjustment device 21is directly connected to the rotating shaft which changes the set angleθ of the recording medium guiding member 10. The set angle θ of therecording medium guiding member 10 is set by the operation of the angleadjustment device 21. On the other hand, the recording medium thicknessdetecting device 23 detects the thickness of the recording medium andoutputs this information in the form of an electrical signal to thecontrol circuit 22. Based on the resultant signal, the control circuit22 outputs an electrical signal to the angle adjustment device 21 sothat it sets the predetermined angle θ in accordance with the thicknessof the recording medium 4. The angle θ is set in accordance with thethickness of the recording medium 4, so that the length of the contactarea 9 is set in accordance with this thickness. In the embodiment, thepaper thickness detecting device 23 is of the light transmittingdetecting type. It is placed at the exit portion of the recording medium4 held by a paper feed hopper (not illustrated) provided in a recordingapparatus including the fixing device of the invention built therein.Instead of using the recording medium thickness detecting device 23, anoperator of the recording apparatus may input the recording mediumthickness by means of an operator panel. According to the embodiment,the thickness of the recording medium is automatically detected and thelength of the contact area is set based on the detected thickness, sothat the right fixing strength can be always provided.

The present invention provides a fixing device capable of preventingimage slippage from occurring and increasing the fixing strength,without the use of a preheater. This is made possible by a contactregion where the recording medium comes into contact with the heatroller after it has passed through the nipping and fusing section,whereby the unfixed toner is formed into a semi-fixed toner, whoselength is changed according to the recording medium thickness.

In addition, the fixing device of the present invention is capable ofpreventing fixing failure at the central portion of the heat roller byincreasing the thermal conductivity of the core metal of the heat rollerand by controlling temperature drop at the central portion of the heatroller when printing is carried out successively for a long period oftime.

Further, the fixing device of the present invention is capable ofpreventing fixing failure at the central, portion of the heat rollerbecause the temperature of the heat roller is controlled when a largeamount of heat is provided to its central portion which consumes a largeamount of heat, and temperature drop of the central portion of the heatroller is controlled when printing is performed successively for a longperiod of time.

Still further, the fixing device of the present invention is capable ofpreventing misprinting and image loss caused by toner stick by purposelyincreasing the time required for the recording medium, after beingsubjected to fixing, to reach the stacker, which results in sufficientcooling of the recording medium.

What is claimed is:
 1. A fixing device in which a recording medium to betransported is nipped to fix a toner image formed thereon, said fixingdevice comprising:a pair of rotatable fixing rollers which face eachother, with at least one of the rollers internally provided with aheating element, said pair of fixing rollers press-contacting therecording medium to form a nipping and fusing section for the tonerimage; and recording medium guiding means which is provided at thedownstream side following the nipping and fusing section formed by thefixing rollers and which change the length of contact area of therecording medium to the outer periphery of the fixing roller, internallyhaving built therein a heating element, in accordance with the recordingmedium thickness.
 2. A fixing device according to claim 1, wherein thefixing roller with heating element is an aluminum hollow cylinder, whichis called a core metal, whose outer periphery is coated with a partinglayer of fluorocarbon resin material.
 3. A fixing device according toclaim 2, wherein the fixing roller with heating element has at least oneheat pipe as heating element embedded in the interior of the cylindricalcore metal in the longitudinal dimension.
 4. A fixing device accordingto claim 3, wherein said recording medium guiding means has a controlmeans for setting the rotating angle of its supporting arm to apredetermined value in accordance with the recording medium thickness.5. A fixing device according to claim 3, wherein a circuitous guidingmember for allowing cooling of the recorded medium is provided alongwith said recording medium guiding means at the downstream side in thetransporting direction of the recording medium between said recordingmedium guiding means and a stacker into which is discharged recordedmedium.
 6. A fixing device according to claim 3 which is used as atleast one component of a recording apparatus.
 7. A fixing deviceaccording to claim 2, wherein the fixing roller with heating element isinternally provided with a plurality of heating elements each having adifferent heating value distribution, in which detectors are provided soas to face the roller for detecting the surface temperature of theroller at positions corresponding to those section of the heatingelements where the heat value of each heating element is maximum, so asto control the operation of the heating elements and thereby to maintainthe roller surface temperature at a predetermined value.
 8. A fixingdevice according to claim 7, wherein said recording medium guiding meanshas a control means for setting the rotating angle of its supporting armto a predetermined angle in accordance with the recording mediumthickness.
 9. A fixing device according to claim 7, wherein a circuitousguiding member for allowing cooling of the recorded medium is providedalong with said recording medium guiding means at the downstream side inthe transporting direction of the recording medium between saidrecording medium guiding means and a stacker into which is dischargedthe recorded medium.
 10. A fixing device according to claim 7 which isused as at least one component of a recording apparatus.
 11. A fixingdevice according to claim 1, wherein the fixing roller with heatingelement has a halogen lamp as heating element in the hollow of thecylindrical core metal.
 12. A fixing device according to claim 1,wherein said recording medium guiding means has a recording medium guideroller and a rotatable supporting arm, in which the rotating angle ofits supporting arm can be set to a predetermined value in accordancewith the recording medium thickness.
 13. A fixing device according toclaim 12, wherein said recording medium guiding means has a controlmeans for setting the rotating angle of its supporting arm to apredetermined value in accordance with the recording medium thickness.14. A fixing device according to claim 1, wherein a circuitous guidingmember for allowing cooling of the recorded medium is provided alongwith said recording medium guiding means at the downstream side in thetransporting direction of the recording medium between said recordingmedium guiding means and a stacker into which is discharged recordedmedium.
 15. A fixing device according to claim 1, which is used as atleast one component of a recording apparatus.
 16. A fixing deviceaccording to claim 1, wherein the fixing roller with heating element isa copper hollow cylinder or core metal, whose outer periphery is coatedwith a parting layer made of fluorocarbon resin material.
 17. A fixingdevice according to claim 16, wherein the fixing roller with heatingelement has nickel plated along the outer surface of the copper hollowcylinder or core metal.
 18. A fixing device according to claim 16,wherein said recording medium guiding means has a control means forsetting the rotating angle of its supporting arm to a predeterminedvalue in accordance with the recording medium thickness.
 19. A fixingdevice according to claim 16, wherein a circuitous guiding member forallowing cooling of the recorded medium is provided along with saidrecording medium guiding means at the downstream side in thetransporting direction of the recording medium between the recordingmedium guiding means and a stacker into which is discharged recordedmedium.
 20. A fixing device according to claim 16 which is used at leastas one component of a recording apparatus.
 21. A fixing method in whicha recording medium to be transported is nipped to fix a toner imagethereon, said method comprising the steps of:fusing the toner bypress-contacting the recording medium between a pair of rotatable fixingrollers facing each other with at least one of its rollers provided witha heating element at its interior; and changing the length of a contactarea of the recording medium to the outer periphery of the fixing rollerwith heating element in accordance with the recording medium thicknessby means of a recording medium guiding means provided at the downstreamside following the nipping and fusing section formed by the fixingrollers.
 22. A fixing method according to claim 21, wherein in the stepof changing the length of the contact area of the recording medium tothe outer periphery of the fixing roller with heating element, therotating angle of a supporting arm of the recording medium guidingmeans, having a recording medium guiding roller and the rotatablesupporting arm, is set in accordance with the recording mediumthickness.
 23. A fixing method according to claim 21 further comprisingthe step of allowing cooling of the recorded medium by providing acircuitous guiding member between the recording medium guiding means anda stacker into which is discharged the recorded medium.
 24. A fixingmethod according to claim 21, wherein in the step of press-contactingthe recording medium between a pair of fixing rollers which face eachother, there is used a fixing roller, internally provided with aplurality of heating elements each having a different heating valuedistribution along the longitudinal dimension of the roller, and whereintemperature detectors are provided so as to face the roller at positionscorresponding to those sections of the heating elements where the heatvalue is maximum to detect the surface temperature of the roller andthereby to control the operation of the heating elements, whereby theroller surface temperature is maintained at a predetermined value.
 25. Afixing device in which a recording medium to be transported is nipped tofix a toner image formed thereon, said fixing device comprising:a pairof fixing rollers, having a rotatable heat roller and a back-up rollerwhich face each other, which form a nipping and fusing section where therecording medium is press-contacted for toner fixing, said heat rollerbeing an aluminum hollow cylinder or core metal whose outer periphery iscoated with a parting layer of fluorocarbon resin material and has atleast one halogen lamp as heating element in its interior, withtemperature sensors for detecting the surface temperature of the rollerprovided in a plurality of places including at least the roller endportion to control the operation of the halogen lamp so that itmaintains the roller surface temperature at a predetermined value; arecording medium guiding member for changing the length of contact areaof the recording medium to the outer periphery of the heat roller inaccordance with the recording medium thickness, which is provided at thedownstream side following the nipping and fusing section formed by thepair of fixing rollers, said recording medium guiding member made up ofa recording medium guiding roller and a rotatable supporting arm inwhich the rotating angle of its supporting arm can be set to apredetermined value in accordance with the recording medium thickness;and a circuitous guiding member which is provided at the downstream sidefrom the recording medium; guiding member between this guiding memberand a recording medium stacker. .Iadd.
 26. A fixing device in which arecording medium to be transported is nipped to fix a toner image formedthereon, said fixing device comprising a pair of rotatable fixingrollers which face each other, with at least one of the fixing rollersbeing internally provided with heating elements, said pair of fixingrollers press-contacting the recording medium to form a nipping andfusing section for the toner image, wherein the at least one fixingroller with heating elements is internally provided with a plurality ofheating elements each having a different heating value distribution, atleast one detector is provided so as to face the at least one fixingroller for detecting the surface temperature of the at least one fixingroller at positions corresponding to at a section of at least oneheating element where the heat value of each heating element is maximum,so as to control the operation of the heating elements and thereby tomaintain the surface temperature of the at least one fixing roller at apredetermined value, and further comprising recording medium guidingmeans provided at the downstream side following the nipping and fusingsection formed by the fixing rollers and for changing the length ofcontact area of the recording medium to the outer periphery of the atleast one fixing roller in accordance with a recording medium thickness..Iaddend..Iadd.27. A fixing device in which a recording medium to betransported is nipped to fix a toner image formed thereon, said fixingdevice comprising a pair of rotatable fixing rollers which face eachother, with at least one of the fixing rollers being internally providedwith heating elements, said pair of fixing rollers press-contacting therecording medium to form a nipping and fusing section for the tonerimage, wherein the at least one fixing roller with heating elements isinternally provided with a plurality of heating elements each having adifferent heating value distribution, at least one detector is providedso as to face the at least one fixing roller for detecting the surfacetemperature of the at least one fixing roller at positions correspondingto at a section of at least one heating element where the heat value ofeach heating element is maximum, so as to control the operation of theheating elements and thereby to maintain the surface temperature of theat least one fixing roller at a predetermined value, wherein a pluralityof detectors are provided so as to face the at least one fixing rollerfor detecting the surface temperature of the at least one fixing rollerat positions corresponding to the sections of the heating elements wherethe heat value of each heating element is maximum, so as to control theoperation of the heating elements and thereby to maintain the rollersurface temperature at the predetermined value, further comprisingrecording medium guiding means provided at the downstream side followingthe nipping and fusing section formed by the fixing rollers and forchanging the length of contact area of the recording medium to the outerperiphery of the at least one fixing roller in accordance with arecording medium thickness. .Iaddend.